mkbuild should change bl_info['warning'] for non-RC builds

[tngreene]

I'm guessing someone looks at that when selected an addon, I do to try and seperate different versions in the same folder. It could be pretty useful. I'll put a yellow in review label on this because it is gold plating. It wouldn't be very hard to do, probably 2 seconds since it already replaces bl_info['minor'] and etc.